1. Field of the Invention
The present invention relates to an apparatus for detecting the amount of liquid nitrogen present for cooling a semiconductor radio detector.
A semiconductor radio detector is used in a plating composition analyzer and a steel composition discriminator. This detector has a detecting element which is a semiconductor, produced by driving lithium into a silicon substrate. However, this detecting element is sensitive to temperature, and in particular lithium comes out of the silicon substrate at a normal temperature, so that the detector can no longer be used. Therefore, during operation, the detector must always be cooled to the appointed temperature by means of liquid nitrogen housed in a Dewar vessel, as disclosed in Japanese Utility Model Publication No. 30312/1978. Since the semiconductor radio detector is expensive, it is necessary to constantly check the amount of liquid nitrogen present for cooling said semiconductor radio detector, so as to be able to supplement the liquid nitrogen before it is used up.
2. Description of the Prior Art
The prior art uses three types of methods for detecting the liquid nitrogen:
(1) a method of detecting nitrogen within a Dewar vessel by sensing the presence of a radio-isotope; (2) a method of detecting the liquid level by utilizing the reflection of ultrasonic waves of light from the liquid; and (3) a gravimetric detection method utilizing a load cell.
However, the method (1) is unsuitable for a small-sized Dewar vessel used in a semiconductor radio detector, and is expensive, method (2) requires the provision of a detector for detecting the liquid level in the Dewar vessel, and there is the possibility that the detector will be broken due to freezing, and the detector is expensive; and method (3) has the disadvantage that the load cell is not sensitive to a certain definite load and it is expensive.